1. TECHNICAL FIELD
The present invention relates generally to joining end portions of elastomeric sheet material. Particularly, the present invention relates to joining end portions of elastomeric sheet material having substantially parallel extending reinforcing members.
2. DESCRIPTION OF THE PRIOR ART
A typical pneumatic radial tire includes a carcass ply which is made from at least one strip of elastomeric sheet material. The elastomeric sheet material has a plurality of parallel extending reinforcing members. Each reinforcing member of the carcass ply is contained in a plane extending substantially radially of the axis of rotation of the tire. End portions of the strip of elastomeric sheet material are overlapped to form a joint
The joint has a length taken in a direction perpendicular to the extent of reinforcing members. The length of the joint is typically in the range of approximately one-quarter of an inch to one inch. The joint also has a thickness taken in direction normal to the length. The thickness of the joint is substantially double the thickness of the elastomeric sheet material. The joint may cause uniformity variations in an area of the tire where the double thickness joint extends under the bead cores.
The joint also has a density of reinforcing members which is approximately double the density of reinforcing members in any other area of the carcass ply. This results in an area of the carcass which is approximately twice the strength of any other area of the carcass. Strength is defined as resistance to expansion of the carcass in the radial direction due to inflation pressure in the tire. The double strength of the joint may result in a visible sidewall indentation in the tire when the tire is inflated. Furthermore, when the tire is tested for uniformity, a variation from the applied test load is often attributable to the double strength of the joint area of the carcass.
U.S. Pat. No. 4,466,473 discloses one end portion of a strip of fiber reinforced elastomeric sheet material which is cut at several locations. The cut extends across the reinforcing fibers for a distance approximately equal to the length of the joint. The cut end portion of the strip contributes substantially no strength to the joint. Thus, the joint has a strength which is substantially equal to the strength of any other area of the tire carcass.
U.S. Pat. No. 4,261,393 discloses a carcass ply for a radial tire including two or three strips of reinforced elastomeric sheet material joined together at end portions. The end portions of the strips of elastomeric sheet material are specially made for the purpose of forming a joint. In one embodiment of U.S. Pat. No. 4,261,393, end portions of each strip used to form a joint contain reinforcing fibers of a smaller diameter than the fibers in an intermediate portion of the strip. The smaller diameter reinforcing fibers have a proportionately lower tensile strength than the reinforcing fibers in the intermediate portion. When the end portions form a joint, the joint has a greater density of fibers than the intermediate portion but the joint has substantially the same strength as the intermediate portion.
In another embodiment of U.S. Pat. No. 4,261,393, the end portions of each strip of elastomeric sheet material contain reinforcing fibers of a diameter equal to the reinforcing fibers in the intermediate portion of the strip. However, adjacent reinforcing fibers in the end portions are spaced apart a greater distance than adjacent reinforcing fibers in the intermediate portion. When the end portions form a joint, reinforcing fibers from one of the end portions are located between reinforcing fibers of the other end portion. The resulting spacing between adjacent reinforcing fibers in the joint is approximately the same as the spacing between adjacent reinforcing fibers in the intermediate portion. Thus, the strength in the joint is substantially equal to the strength in the intermediate portion.